1. Field of the Invention
The present invention relates to vacuum interrupters, that is, electrical circuit interrupting devices comprising a pair of cooperating contacts in an evacuated enclosure, the contacts being relatively movable between closed and open positions to complete or interrupt an electric power circuit under normal load current and fault conditions. More specifically, the invention relates to the manufacture of contact materials which find use in such vacuum interrupter devices.
2. Description of the Prior Art
In vacuum interrupters, the ambient conditions under which arcing occurs between the contacts when they are separated to interrupt current and the mechanism of arc extinction in vacuum interrupters, is somewhat different from the mechanism of arc extinction in other types of circuit interrupters in which the arcing occurs in a medium such as insulating oil or gas. Consequently, the tendency for the contacts to weld together when operated in the vacuum is much more severe. Therefore, the choice of contact materials for other types of circuit interrupters and related devices such as spark gaps is not relevant to the choice of contact materials for vacuum interrupters.
Heretofore, refractory metals such as tungsten, molybdenum and their carbides have been successfully used for the contacts of vacuum interrupters of relatively low current interrupting capability. In particular, such contacts are characterized by porous matrix of refractory metal particles metallurgically bonded together usually comprising a sintered compact of interrupting particles and the interstices of the matrix are infiltrated with a high conductivity metal usually copper which characterically possesses a lower melting and boiling point as well as higher electrical and thermal conductivities than that of the matrix material. Such refractory metal matrix provides high mechanical strength and good erosion resistance. Moreover, the matrix metal has little tendency to weld and being of low ductility also maintains smooth contact surfaces and consequently high open circuit dielectric strengths. The presence of the infiltrant reduces the current chopping level which is excessively high in contacts made of refractory metals. The current inerrupting ability of vacuum interrupters having contacts of such infiltrated refractory matrix materials is limited however because of excessive thermionic emission from the refractory constituent at high currents.
One of the solutions posed to these problems resulted in the development of vacuum interrupters of higher current interrupting capability and departed from the use of infiltrated matrix contact materials. This solution involved the use of alloys in which a major constituent metal is alloyed with a minor constituent the latter of which forms brittle films at the grain boundries between the crystals of the major constituent. Typical of such alloys is the composition represented by the copper-bismuth system. With this alloy material, the excessive thermionic emission associated with refractory metals is avoided and the effects excessive weld tendency and ductility of the high conductivity metals are relieved by what has been termed intercrystalline weaknesses. With these intercrystalline weaknesses, intercontact welds are easily broken without drawing spikes from the contact surfaces during circuit interruption.
Moreover, the intercrystalline weakness makes the contacts mechanically inferior to those contacts employing the infiltrated matrix material of low ductility. Consequently, since they are mechanically weak throughout the structure, the material does not have the ability of the matrix material to retain contact profile during operation. As a result, separation of the contact material tends to rupture intercrystalline boundries both near the original interface and further into the body of the material. This mechanical weakness also places many limitations on the mechanical design of the contacts, which ideally is determined by the plasma physics of the arc.
Another solution proposed to alleviate these problems involved the utilization of the better parts of the two previous solutions. This solution involves making the contact materials from parts which constituted a porous matrix of metal particles, metallurgically bonded together, and in which the interstices of the matrix material are infiltrated with another metal of lower melting and boiling points and high electrical and thermal conductivies. The matrix material usually comprising particles of a low conductivity but fairly high melting point metal, that is, the metal had a melting point higher than the high conductivity metal which was used as the infiltrant and which forms a precipitation alloy component with the infiltrated metal but does not otherwise form any degree of alloys having solid solubility of the one metal in the other. As a result, in the surface region of the contacts which are melted by arcing, the precipitation alloy forms and on subsequent cooling recrystallizes to reestablish the infiltrated matrix structure with the particles of the sintered matrix structure remaining intact throughout the arc operation.
Typically, these contacts were made by sintering particles of a metal having a high melting point, such as for example chromium and infiltrating the same with copper or silver so as to form a unitary structure. Thus, when copper and chromium are used, the matrix chromium metal is soluble to a substantial extent in the copper metal, when the latter is liquid and heated to a sufficiently high temperature, but the solid solubility is quite low usually being of the order of less than 1 percent.
During operation, a precipitation alloy component will be formed in the contact surface region and the surface structure will be refined and improved as a result of reprecipitation on subsequent cooling. It has been found, however, that when such contact materials are utilized in high current vacuum interrupters, the interruption performance becomes markedly improved with high power testing. Examination of the contact surfaces of chromium-copper contacts materials reveals that the surface becomes altered due to excessive arcing and melting when the interrupter passes several current zeros before interruption. It has been found, however, that this melting and alteration of the metallic structure does not appear to degrade the interruption performance but in fact appears to enhance its performance.
As a result, one of the usual techniques employed with the utilization of these chromium copper contact materials evolved a rather lengthly and costly seasoning procedure in which the contact materials are employed and utilized in a number of high current interruptions in order to season the contact surface.
The method of the present invention utilizes a different method of manufacturing said contact materials and as a result it has been found that the heretofore found necessary seasoning procedure of such contact materials can be virtually eliminated and the materials embodied into a vacuum interrupter without prior high current power testing and seasoning procedures being involved.